The unstable bioregulator thromboxane A2 (TXA2) is generated from the prostaglandin (PG) endoperoxide PGH2 by an enzyme called thromboxane synthetase. Thromboxane synthetase is present in many tissues including blood platelets, lung, spleen and brain. Simultaneous with the formation of TXA2 is the formation of C-17 hydroxy fatty acid (HHT) from PGH2. We have investigated this process with emphasis on kinetics and inhibitors. We have shown that HHT (12-Hydroxy-5,8,11-hepadecenoic acid) is not a breakdown product of TXA2 but that TXA2 breaks down exclusively into thromboxane B2. Moreover, our results indicate that thromboxane synthetase is not an isomerase but rather a bimolecular reaction is involved in the formation of TXA2 and perhaps HHT. Also, we have preliminary evidence that there is another unstable intermediate formed from PGH2 which breaks down to HHT. The existence of another intermediate poses interesting experimental questions concerning possible biochemical and biological activities.